Ezra 2:62
These people searched for their ancestral registrations but they couldn't be located. Accordingly, they were assigned an "unclean" status and couldn't be priests.
Numbers 3:10
Appoint Aaron and his sons so that they are to take responsibility for their priesthood. Any unauthorized person who approaches it is to be put to death."
Leviticus 21:21-23
None of the descendants of Aaron the priest who has a defect is to approach to bring offerings of the LORD made by fire, since he has a defect. He is not to approach to offer the food of his God.
Numbers 16:39-40
So Eleazar the priest took the bronze censers that had been burned and beat them into metal plates for the altar,
Numbers 18:7
Now you and your sons with you are to maintain your priestly duties and all matters that concern the altar and what is housed within the veil. You are to perform these services. I'm giving you the priesthood as a gift of service, but any unauthorized person who approaches is to be put to death."
Ezekiel 44:10-14
But the descendants of Levi, who went far away from me when Israel abandoned me, who left me to follow their idols, are to bear the punishment of their iniquity.